The present invention relates to a heat transfer sheet and, more particularly, seeks to provide a heat transfer sheet capable of making a recorded image excelling in density of developed colors, definition and various fastness properties, and in particular, storability and resistance to discoloration/fading.
Heretofore, various heat transfer techniques have been known in the art, including sublimation type transfer systems wherein a sublimable dye is carried on a substrate sheet such as paper to make a heat transfer sheet, which is then overlaid on an imageable material dyeable with a sublimable dye, for instance, a woven fabric made of polyester, to apply heat energy in the form of a pattern from the back side of the heat transfer sheet, thereby transferring the sublimable dye to the imageable material.
More recently, there have been proposed techniques for making various full-color images on paper or plastic films with the above sublimation type of heat transfer systems in which thermal heads of printers are used as heating means to transfer three-, four- or more-color dots to image-receiving sheets by heating for a very short-time, thereby reproducing full-color images of manuscripts with the multicolor dots.
Because the coloring materials used are dyes, the images thus formed are very clear and because of their excellent transparency, the obtained images are improved in reproducibility and gradation of neutral tints, are equivalent to those achieved by conventional offset or gravure printing, and are comparable in quality to full-color photographic images.
However the most important problems with the above heat transfer systems are the density of the developed colors, and the storability and resistance to discoloration/fading of the formed images.
In fast recording, heat energy is required to be applied within a time as short as a fraction of a second. However, no image of sufficient density can be obtained since sublimable dyes and image-receiving sheets are not well heated within such a short time.
In order to cope with such high-speed recording, sublimable dyes excelling in sublimability have been developed. However, problems with such dyes of excellent sublimability are that after transfer, they pass into the imageable materials or bleed onto their surfaces with time, generally because of their low molecular weight. As a consequence, the images, once formed, become diffused or blurred, or otherwise contaminate surrounding articles.
Even when a sublimable dye having a relatively high molecular weight is used to avoid such problems, an image of satisfactory density cannot be obtained since its rate of sublimation is too slow for such fast recording as mentioned above.
Generally, the obtained images are inferior in light resistance to those obtained with pigments, because of being formed of dyes, and therefore posing problems that premature fading or discoloration takes place upon direct exposure to sunlight. Such light resistance problems may be solved to some extent by adding UV absorbers or antioxidants to the dye-receiving layers of image-receiving sheets.
However, discoloration/fading problems are also caused for other reasons, for instance, when the images are exposed to indoor light or even while they are slipped in albums or encased in cases or form parts of books, all protected against the direct rays of the sun. Such discoloration/fading problems occurring indoors and in dark places can never be solved by using generally available UV absorbers or antioxidants.
A main object of the present invention is therefore to provide a heat transfer sheet which can be applied to a heat transfer process using a sublimable dye to make a clear image which is not only of sufficient density but which also excels in various fastness properties, esp., storability and resistance to discoloration/fading.